Ian T. Riley

Phytoecdysteroids: a novel defense against plant-parasitic nematodes.

The phytoecdysteroid, 20-hydroxyecdysone (20E), is a major molting hormone of invertebrates possibly including nematodes. As 20E is inducible in spinach, the defensive role against plant-parasitic nematodes was investigated. The effects of direct application on nematodes was assessed by treating cereal cyst nematode, Heterodera avenae, juveniles with concentrations of 20E from 8.2 × 10−8 to 5.2 × 10−5 M before applying to Triticum aestivum growing in sand. H. avenae, Heterodera schachtii (sugarbeet cyst nematode), Meloidogyne javanica (root-knot nematode) and Pratylenchus neglectus (root lesion nematode) were treated with 5.2 × 10−5 20E and incubated in moist sand. To test the protective effects of 20E in plants, the latter three nematodes were applied to Spinacia oleracea in which elevated concentrations of 20E had been induced by methyl jasmonate. Abnormal molting, immobility, reduced invasion, impaired development, and death occurred in nematodes exposed to 20E either directly at concentration above 4.2 × 10−7 M or in plants. Phytoecdysteroid was found to protect spinach from plant-parasitic nematodes and may confer a mechanism for nematode resistance.

Evolution of the gall-forming plant parasitic nematodes (Tylenchida: Anguinidae) and their relationships with hosts as inferred from Internal Transcribed Spacer sequences of nuclear ribosomal DNA.

Phylogenetic relationships among gall-forming plant parasitic nematodes of the subfamily Anguininae are reconstructed by maximum parsimony and maximum likelihood analyses. Sequences of the ITS of rDNA from 53 populations and species of gall-forming nematodes and five populations of the Ditylenchus dipsaci species complex were analysed. The phylogenetic trees strongly support monophyly of the genus Anguina and show nonmonophyly for the genera Mesoanguina and Heteroanguina. Morphological and biological characters are generally congruent with the anguinid groups identified in the rDNA phylogeny. Analyses of evolution of different gall types among anguinids reveal that there are apparent evolutionary trends in gall evolution: from abnormal swelling and growth of infested plant organs toward small localised galls, and from infestation of vegetative toward generative organs. Our study demonstrates that the main anguinid groups are generally associated with host plants belonging to the same or related systematic groups. The comparison of the ITS phylogenies of anguinids parasitising Poaceae and their host grasses shows a high level of cospeciation events.

New pathotypes of Heterodera avenae (cereal cyst nematode) from winter wheat in Zhengzhou, Henan, China

A growing recognition of the importance of the cereal cyst nematode, Heterodera avenae in winter wheat in China has highlighted the need to better understand local pathotype diversity, as this knowledge is needed to effectively deploy host resistance. Two populations of H. avenae, one each from Xushui and Xingyuang, villages near Zhengzhou, Henan, on the Huang Huai flood plain where 50% of China’s wheat is grown, were typed using 23 standard international differentials and a common local cultivar Wenmai 19. These populations were found to be previously undescribed pathotypes. It is proposed that the pathotype code, Ha43, be applied to the population from Xushui, which was consistently characterised over two seasons and differs from the most similar pathotype, Ha13, by being avirulent in oat cv. Sivan and wheat cvv. Loros and Iskamish K-2-light. Wenmai 19 was found to be susceptible to both populations. Australasian Plant Pathology Society 2010

Genetic Analysis of Plant Pathogenic Bacteria in the Genus Clavibacter Using Allozyme Electrophoresis

Summary: Allozyme electrophoresis was used to examine the relationships among species of Clavibacter, a genus of bacteria mostly causing gummosis in various graminaceous hosts. Allelic profiles were determined at 20 enzyme loci for 28 strains representing C. iranicum, C. rathayi, C. tritici, three subspecies of C. michiganense, Clavibacter sp. from Lolium rigidum, and “Corynebacterium agropyri”. The genetic differences between species were at least 70% and between strains within a species up to 35% except for the subspecies of C. michiganense, which differed by up to 75%. The implications of these results for the classification of these Clavibacter spp. is discussed.

The association of necrotrophic fungal pathogens and plant parasitic nematodes with the loss of productivity of annual medic-based pastures in Australia and options for their management

Annual medics (annual Medicago spp.) are often an integral component of dry-land farming and cropping systems worldwide, including significant areas of the winter rainfall regions of southern Australia. Necrotrophic fungal pathogens frequently dominate such areas because of the ease of survival of these trash-and soilborne pathogens on infested residues over the relatively dry summer period. Necrotrophic fungal pathogens, in general, are favoured by the nutrient impoverished soils across many parts of these regions, which provide little microbial buffering against these pathogens. This review first, outlines the major and most widespread diseases caused by necrotrophic fungal pathogens and plant parasitic nematodes; second, defines the association of these pathogens with the loss of productivity of annual medic-based pastures; and finally, investigates the spectrum of control options for their management. Important necrotrophic foliar fungal pathogens include Phoma medicaginis, Colletotrichum trifolii, Leptosphaerulina trifolii, Pseudopeziza medicaginis, Stemphylium botryosum, S. vesicarium and Stagonospora meliloti. Necrotrophic foliar fungal pathogens, for example Phoma medicaginis, are also known to stimulate production of phyto-oestrogenic compounds to high levels that can adversely affect ovulation rates in sheep. There are also necrotrophic fungal root pathogens in Australia that have been associated with significant decline in productivity and pose such a serious threat to annual medic pastures such that reseeding is required. In particular, Rhizoctonia solani and various Fusarium species such as F. avenaceum, F. acuminatum, F. culmorum, F. graminearum and F. lateritium, and various Pythium species, in particular P. irregulare, P. ultimum and P. spinosum, are of concern. Another important soilborne and root-attacking necrotrophic pathogen is Phytophthora clandestina. The association of Fusarium spp. with annual medic roots, crowns and burrs in Australia is cause for additional concern as some of them are responsible for the production of deleterious mycotoxins. Plant parasitic nematodes are important as a potential yield limiting factor in annual medics. Approaches to disease management include strategies that have been utilised to varying degrees for the control of necrotrophic foliar and root pathogens in annual medic pastures. This paper reviews the current literature on the topic and provides an assessment of options available for their management. In particular, host resistance offers the most cost-effective, longterm control, particularly as some useful resistance to several of these pathogens has been identified. One or more cultural control strategies, including grazing, fertiliser application, rotations and seed health, offer further opportunities for restricting losses in annual medics from diseases caused by necrotrophic fungi and/or plant parasitic nematodes, especially if applied as an integrated management strategy with only minor reliance on fungicides. As many of these foliar and soilborne fungal pathogens and parasitic nematodes can also threaten crops grown in rotation, these pathogens pose a wider threat to the farming systems employed across southern Australia and reduce the potential for providing a ‘disease break’. In addition, the role played by these pathogens is likely far wider and of greater adverse impact than previously considered as a consequence of their potential for causing mycotoxin and/or phyto-oestrogen production. The full array of losses, not just herbage and seed yields, needs to be considered for the more important fungal and nematode pathogens. Genetic resistance to individual pathogens provides the best option for management. The success and outcome with sourcing resistance in other annual pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover. pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover.

Laimaphelenchus preissii sp nov (Nematoda : Aphelenchina) from native pine Callitris preissii in South Australia

Summary Laimaphelenchus preissii sp. nov. is described from bark of the Australian native conifer, Callitris preissii from Burdett, South Australia. This is the first record of Laimaphelenchus from Australia. The new species is characterised by the possession of a unique tail structure, with a single tubercle with many small projections, and the male has a bursa and two pairs of caudal papillae.

Severity of root rot in mature subterranean clover and associated fungal pathogens in the wheatbelt of Western Australia

Pasture decline is considered to be a serious challenge to agricultural productivity of subterranean clover across southern Australia. Root disease is a significant contributing factor to pasture decline. However, root disease assessments are generally carried out in the early part of the growing season and in areas predominantly sown to permanent pastures. For this reason, in spring 2004, a survey was undertaken to determine the severity of root disease in mature subterranean clover plants in pastures located in the wheatbelt of Western Australia. DNA-based soil assays were used to estimate population density in the soil of a variety of soil-borne pathogens known to commonly occur in the Mediterranean-type environments of southern Australia. The relationships between severity of disease on tap and lateral roots and root diameter, root length, nodulation, and total rainfall were determined. The survey showed, for the first time, that severe root disease is widespread in spring across the wheatbelt of Western Australia. There was a positive correlation between rainfall and tap root disease, and between tap root disease and average root diameter of the entire root system. Despite the high levels of root disease present across the sites, the DNA of most root disease pathogens assayed was detected in trace concentrations. Only Pythium Clade F showed high DNA concentrations in the soil. DNA concentrations in the soil, in particular for Phytophthora clandestina and Rhizoctonia solani AG 2.1 and AG 2.2, were higher in the smaller autumn sampling in 2006. This study suggests that the productivity of subterranean clover-based pastures is severely compromised by root rot diseases throughout the growing season in the wheatbelt of Western Australia.

Anguina tritici is a potential vector of Clavibacter toxicus

The ability of Anguina tritici to act as a vector for Clavibacter toxicus in wheat was determined in a pot experiment. Using galls colonised with C. toxicus from Lolium rigidumas inoculum, gall production by A. tritici was reduced and a small proportion of galls was colonised by C. toxicus. These observations further confirm that the host range of C. toxicus is determined by the availability of a suitable nematode vector. This experimental association of an Anguina and a Clavibacter suggests that there is potential for a non-toxigenic Clavibacter sp. being used for control of Anguina funesta and C. toxicus in Annual Ryegrass Toxicity.

Molecular characterisation of cereal cyst nematodes in winter wheat on the Huang-Huai floodplain of China using RFLP and rDNA-ITS sequence analyses

In response to the recent discovery of Heterodera filipjevi central Henan and the uncertain taxonomic status of Heterodera avenae more widely in China, heteroderid specimens from winter wheat at 21 locations in Henan and adjacent provinces were subjected to RFLP and rDNA-ITS sequence analysis. H. filipjevi was found in six locations in Henan, including two mixed with H. avenae; H. avenae being found at all other locations. A new RFLP profile type was found for H. filipjevi, three new types for H. avenae and the Australian type (Heterodera australis) were found for the first time in China (at two locations). Otherwise, H. avenae and H. filipjevi were of RFLP types previously reported in China. Phylogenic analysis of the rDNA sequences showed H. filipjevi in China was less diverse than H. avenae, with greatest similarity to specimens from Italy and the USA, which is consistent with a more recent introduction. In contrast, H. avenae in China was clearly distinct from H. avenae found elsewhere, except for the discovery of the Australian types. Although the Australian types clustered together, this fell within the variation found for the remainder of the specimens from China, which may represent a single species. These data reveal additional genetic diversity within the two cereal cyst nematode species in China, which is likely to have implications for the development of their control by host resistance.

Relationship of rainfall, cultural practices, soil and plant nutrients, and seedling survival with root disease and parasitic nematode numbers in annual Medicago spp. pastures

Surveys were conducted for annual Medicago spp. (medic) pastures in the grain belt of south-west Western Australia during spring 1996 and winternspring 1997 to determine the relationship of rainfall, cultural prac- tices, soil and plant nutrients, and seedling survival with severity of root disease and numbers of parasitic nema- todes. Medic pasture was sampled on 116 farms. Most pastures consisted of a single medic variety, viz. Serena, Santiago, Cyprus, or Caliph, whereas about 33% of sites had mixed varieties. Regression analyses showed that high rainfall and application of phosphorus fertilisers were correlated with increased severity of rot in medic tap roots. Crop history and medic variety were not related to the level of root rot. Numbers of Pratylenchus in medic roots were not correlated with the level of tap or lateral root rot, medic variety, rainfall, or with the application of insec- ticide, fertilisers, or herbicides. Soil with relatively high levels of P, NO3 , or Fe was associated with an increased level of tap root rot. Soils with high pH were associated with reduced tap root rot. Soils with relatively high K were related to severe lateral root rot, wheras relatively high levels of P in soil were associated with reduced lateral root rot. Plants with high levels of tap root rot showed low levels of Mg, whilst low levels of Ca and NO3 n in tissues were related to high levels of lateral root rot. High levels of tap root rot were associated with relatively high levels of total N, K, and S, Cu, Zn, Mn, and NO3 n in plant tissues. Plants with relatively high levels of lateral root rot had rela- tively high levels of Cu in shoots. Of the 116 annual Medicago pastures sampled, only 1% had adequate Mg content and only 19% had adequate Ca content. However, 83% had higher than adequate levels of Cu, 70% had higher than adequate levels of Mn, and all samples showed more than adequate levels of chloride. Experimental sites of M. poly- morpha cv. Serena at 6 farms showed that the percentage survival rate of seedlings was negatively correlated with the severity of tap and lateral root rot in the previous year. These results indicate that in the farms surveyed there is a serious threat to annual medic pastures from root rot fungi. The severity of the disease was partly determined by soil conditions and cultural practices.